This invention relates to tennis rackets, and, more particularly, to a tennis racket which is lighter than conventional rackets yet which is stiffer and has a higher center of percussion than conventional rackets.
Frolow U.S. Pat. No. Re. 31,419 describes a racket which is lighter than the standard weight of about 12 to 14 ounces (including the weight of the strings), has a higher center of percussion, a higher center of gravity, a higher frequency of vibration, and a higher ratio of Ia/Is than conventional rackets. Specific values for the foregoing physical properties are set forth in FIG. 40. FIG. 22 illustrates a racket made from graphite fibers and epoxy which has a non-oval head shape. The upper end of the head is relatively flat and extends somewhat perpendicularly to the longitudinal axis of the racket.
Frolow U.S. Pat. No. Re. 33,372 describes additional features, including a large head racket and a weight in the handle. Specific values for certain physical properties are set forth in FIG. 15.
Kuebler U.S. Pat. No. 4,664,380 describes a racket which has the shape of a dual tapered beam in side elevation (FIG. 7). The widest or thickest part of the racket in side elevation is in the general area of the throat of the racket.
Wilson EPO Pat. No. 317,711 describes a racket having a frequency of the first mode of bending under free-free constraint between 170 Hz and 250 Hz.
Wilson Sporting Goods Co. has sold tennis rackets under the name Hammer which have some of the foregoing properties as shown in Table 1. The Hammer rackets are made from graphite fibers and resin.
TABLE 1 __________________________________________________________________________ Highest Center Moment of Inertia Highest Point At Percussion Weight Balance Si Frequency IY I.sub.2 = END Point Shoulders From Butt Model (Oz.) (In.) (mm) (Hertz) (Oz In.sup.2) (mm) (mm) (In.) __________________________________________________________________________ Wilson Hammer 2.7si 95 9.72 15.35 2.7 198 89 2900 33.4 33.4 19.40 Wilson Hammer 2.7si 110 10.05 15.12 2.7 194 104 2925 38.0 38.0 19.40 Wilson Hammer 4.0si 95 9.72 15.35 4.0 178 89 2900 29.5 29.5 19.40 Wilson Hammer 4.0si 110 10.05 15.12 4.0 176 104 2925 34.5 34.5 19.40 Wilson Hammer 5.0si 95 9.72 15.35 5.0 162 89 2900 27.0 27.0 19.40 Wilson Hammer 5.0si 110 10.05 15.12 5.0 160 104 2925 30.0 30.0 19.40 Wilson Hammer Classic 10.50 14.40 5.2 150 89 2900 22.0 22.0 19.20 5.2si 95 __________________________________________________________________________
The numbers 95 and 110 refer to the area of the string surface--95 square inches and 110 square inches, respectively. The letters si mean stiffness index and refer to the deflection of the racket when the racket extends horizontally and the grip is clamped and a weight of 2.8 kg is suspended from the tip of the racket. A stiffness index of 2.7 means that the tip of the racket deflects 2.7 mm. A stiffness index of 4.0 means that the tip of the racket deflects 4.0 mm. The 2.7 si racket deflects less and is stiffer than rackets with higher si numbers.
Balance refers to the balance point or center of gravity of the racket and is measured from the butt end of the racket.
Frequency refers to the frequency vibration of the first mode of bending under free-free constraint. That frequency is described in Wilson EPO Pat. No. 317,711, and is comparable to the frequency F.sub.1 which is referred to in FIG. 40 and in column 15, lines 50-56 of Frolow U.S. Pat. No. Re. 31,419. FIG. 15 of Frolow U.S. Pat. No. Re. 33,372 also refers to the frequency F.sub.1. The frequency of a racket is related to the stiffness of the racket. A stiffer racket has a higher frequency.
I-Y is the moment of inertia about the longitudinal center line of the racket. I-Y is the same as Ia which is described in Frolow U.S. Pat. No. Re. 31,419.
I-END is the moment of inertia about an axis which extends through the butt end of the racket and perpendicularly to the longitudinal centerline. I-END is the same as Is which is described in Frolow U.S. Pat. No. Re. 31,419.
The Hammer rackets are dual taper beam rackets similar to the shape illustrated in Kuebler U.S. Pat. No. 4,664,380. The term Highest Point in Table 1 refers to the widest dimension of the side elevation of the racket. The term Highest Point at Shoulders refers to the widest dimension of the side elevational of the racket where the yoke which forms the bottom of the head of the racket merges with the side of the head.
Center of Percussion from Butt refers to the center of percussion when measured according to conventional techniques. One such technique is described in Frolow U.S. Pat. No. Re. 31,419 at column 6, lines 27-43. The number for Center of Percussion means the distance from the butt end of the racket.